[2021] This game's stealth...LOL

I don't know... I've lost count of the number of times I've walked right up to a false breakaway door, bashed it open, and startled the zombie that's on the other side, facing me, that was supposed to break out of it to startle me. So I could believe the screenshot is possible in vanilla. Like aY227 alludes, there can be a problem with the individual sleeper volume controlling the sleeper zombie's senses, instead of a systemic problem. But one thing's for sure: the game tries really, really hard to not activate a sleeper zombie early.